1. Technical Field
The invention relates to a surveillance camera system using a plurality of cameras and, more particularly, a surveillance camera system capable of preventing such a situation that captured information of a particularly important local area in a shooting area is lost due to overexposure/underexposure while continuing the shooting under automatic exposure control.
2. Description of the Related Art
As a camera for use in surveillance, normally a digital camera having an automatic exposure controlling function is used. Exposure control is made such that an integrated brightness level in the overall shooting area is set properly. Control of an exposure value is made by adjusting a diaphragm incorporated into an imaging lens, a charge storage time of an image sensor, and a gain of an imaging signal obtained by the image sensor, based on one photometric value derived from the overall shooting area. Such exposure control system requires less burden in terms of both the hardware and the software and is stable in operation. On the other hand, when great variation of brightness is caused in a part of the shooting area, its influence is prone to extend over the full screen. Therefore, this exposure control system has such a problem that since an original target of surveillance is overexposed or underexposed, it becomes difficult to get adequate image information.
In order to address this problem, the following approaches have been known. That is, JP 2001-211383 A discloses when variation in a brightness level in the shooting area can be predicted on a time basis and a season basis, the exposure control is applied by utilizing already-known information in such a manner that the brightness level of a subject which is a target of surveillance is not excessively changed. Also, JP 2003-319248 A discloses that the maximum brightness and the minimum brightness are detected from a shooting screen, then a photometric value is calculated in response to what extent a high brightness area extending from a predetermined high brightness level to the maximum brightness and a low brightness area extending from a predetermined low brightness level to the minimum brightness are contained in the overall shooting screen as an area ratio, and then the exposure control is applied based on this photometric value.
However, the approach disclosed in JP 2001-211383 A is effective for a previously expected environmental change such as change in a solar position, change in weather, or the like, but cannot deal with an artificial or unexpected change. Also, in the approach disclosed in JP 2003-319248 A, in order to evaluate a brightness distribution pattern in the whole screen, it is necessary to acquire brightness information by dividing the shooting screen into pixel units or a plurality of areas. Thus, image processing becomes complicated and a processing time is prolonged. Therefore, a follow-up performance also becomes an issue in the case where the brightness of a part of the shooting screen is changed within a short time. Also, when the type of camera or a shooting magnification ratio is changed in response to the purpose of surveillance, it is necessary to change a photometric program or exposure control program. Thus, there is a drawback in the viewpoint of versatility.